Charles Hagen (1877 - 1925) was a magician and magic magazine publisher. His periodical, Boy Magician, ran for one year (1910), then was renamed American Magician, and was published for one more year.
Hagen was a collector of magic memorabilia, particulary magicians' wands. He published a booklet called "Weird Wands Which Wonder Workers Wielded" in 1917, showcasing his collection.
Earlier, in 1891, he had put together a huge, impressive 300-page scrapbook containing magicians' business cards, autographed letters, programs, handbills and other memorabilia. This scrapbook found its way into Houdini's personal library, and was donated to the Library of Congress upon Houdini's death.
Most likely Houdini and Hagen had a friendship, as they shared so many common interests, and Houdini obtained the scrapbook directly from Hagen.
This envelope was postmarked on April 8, 1919 at Grand Central Station in New York. Houdini celebrated his birthday on April 4; perhaps this envelope contained birthday wishes from his friend, Charles Hagen.